Solar energy collection devices have been developed which are oriented to receive solar radiation and to collect and transform solar radiation to thermal energy using a circulating or recirculating heat collection fluid. The collection fluid flows to a solar radiation collection station where it is heated by the rays of direct or reflected sunlight. The fluid is pumped from the solar collection station to a heat exchanger, where the thermal energy collected is utilized. In one type of heat exchanger for use with solar collectors in which, for example, liquid sodium is the heat collection fluid, the heat exchanger may include coils carrying the liquid sodium proximately positioned to coils carrying water. In this manner the water is heated by the liquid sodium, thereby effectuating transfer of the thermal energy acquired by solar collection to the water.
Solar radiation which is entrapped as thermal energy may be used directly for the purpose of heating water. Alternatively or in addition to the use of a heat exchanger, the heat collection conduit may carry the heat collection fluid to a heat sink. The heat sink may be an insulated storage tank in some applications, such as where the heat collection fluid is water. The tank, in such instances, resembles a tank used in conventional gas or electric hot water heaters. The water heated by solar energy is dispensed from the heat sink as required for use. One embodiment of such a system provides heated water for household use or for a heated swimming pool.
Several different forms of solar energy collection systems are utilized in which a heat transfer fluid is circulated or recirculated to acquire thermal energy and to carry this energy from the solar collector for later release. The most widely used type of solar collector is the panel or flat plate collector. In this type of solar energy collection device a heat collection fluid follows a serpentine path through a tubing system lying substantially in a flat plane. A transparent sheet or membrane may be stretched across the upper surface of the collector and reflectors are sometimes located beneath distinct portions of the tubing to entrap radiation from the sun as thermal energy within the collector and to transfer the incident heat of radiation to a fluid flowing through the tubing. Another type of solar energy collector is a concentrator. In a concentrator, a highly reflective parabolic trough is directed to face the sun. A linearly aligned conduit is positioned to extend along the focal axis of the parabolic trough so that solar radiation incident to the trough is reflected to the linear conduit and is absorbed as thermal energy by a heat transfer fluid circulating therein. While numerous other types of solar energy collection devices have been developed, the flat plate collector and the concentrator are of the greatest commercial significance.
One problem present in connection with solar energy collection devices that employ a heat transfer fluid is the problem of retaining heat collected within the fluid. Once the temperature of the heat collection fluid has been raised to exceed ambient temperature, there is a tendency for the heat collection fluid to radiate thermal energy, thereby losing the solar energy that it has acquired. To compensate for this, some systems have jacketed the heat collection tubing used with a transparent vacuum jacket. The objective of such an arrangement is to allow solar radiation to pass as incident light energy through the transparent vacuum jacket to enter the fluid transfer medium. By transforming the incident solar radiation to thermal energy, rather than maintaining it in the form of light energy, outward radiation from the heat collection fluid is inhibited by the surrounding insulating vacuum. However, several defects exist in connection with this approach. One principal disadvantage of conventional systems is that oftentimes the vacuum jacket fails to maintain a good vacuum seal. This is because the transparent material, usually glass, is sealed to the fluid transfer tubing, typically formed of copper pipe with a blackened outer surface. Because of the differences in coefficients of thermal expansion between the glass jacket and the metallic tubing, the vacuum seal formed therebetween is easily broken when the device is in use. Thus, the vacuum surrounding the fluid transfer tubing is frequently lost or of such a low differential from surrounding ambient pressure that it is ineffective to adequately prevent radiation of thermal energy from the interiorally located tubing.
Another problem that has existed in connection with conventional vacuum jacketing of solar collection conduits is the positioning of the surrounding vacuum jacket in direct contact with the fluid transfer tubing. Thus, while thermal radiation from fluid within the tubing may be inhibited by the surrounding vacuum, the contact of the jacket with the tubing provides a path for conducting heat away from the fluid. That is, heat is transferred by conduction through the structure of the jacket in addition to any heat losses which may exist by virtue of radiation as a result of poor vacuum sealing.
Accordingly, it is an object of the present invention to provide a solar collection conduit which inhibits radiation loss from a heat transfer fluid within a solar energy collection device while at the same time guarding against loss of heat by conduction. This objective is achieved by surrounding the heat collection tubing with a vacuum jacket within an insulation space. This insulation space is typically filled with dead air which is prevented from circulating. Thus, to escape the heat transfer fluid, thermal energy must be conducted from the fluid to the surrounding copper tubing, and must radiate through a dead air space and subsequently through an evacuated chamber before it is lost as a source of energy. This arrangement markedly decreases thermal losses from the fluid transfer medium when contrasted with conventional devices.
A further object of the invention is to provide a conduit construction in which heat transfer fluid tubing is surrounded by an evacuated chamber which avoids vacuum seals between the fluid transfer tubing and the material of which the vacuum jacket is constructed. A vacuum jacket construction according to the present invention thereby avoids the metal-glass interfaces that are so unsatisfactory for maintaining vacuum seals and which have been so prevalent in the prior art. Rather, the present invention employs a vacuum jacket comprised of an inner glass sleeve and an outer glass sleeve. These sleeves may be of several configurations, depending upon the heat transfer fluid tubing configuration employed. Where the heat transfer fluid tubing is constructed to allow fluid to traverse from one side or end of a solar collection panel or reflector trough and to travel across the collector surface to exit at an opposite end, the vacuum jacket is preferrably configured as a longitudinally elongated pair of concentric cylinders joined at either end by opposing halves of a toroidal surface. The length of the cylinders is commensurate with the length of the heat collection fluid tube sections which are surrounded thereby.
A further object of the invention is to provide a fluid transfer conduit for use in a solar energy collection system which is equipped with a vacuum chamber surrounding central fluid conducting tubing located within the interior confines of the vacuum jacket within an insulation space between the inner wall of the jacket and the fluid transfer tubing. Moreover, this heat conserving conduit is preferrably protected externally of the solar radiation receiving section by armored sheathing located thereabout. Such armor sheathing may take the form of steel or copper tubing positioned to coaxially envelop the vacuum jacket. An additional vacuum layer may be provided in such an embodiment where the steel or copper tubing is itself constructed of multiple walls which define an additional evacuated area therebetween. Again, sealing between disimiliar materials is avoided, since no seal exists between the glass vacuum jacket and the surrounding metal vacuum jacket. Instead, the glass walls of the interior glass jacket are sealed together while the metal walls of the exterior metal jacket are either sealed to each other or to interface connections.
An additional object of one embodiment of the invention is a solar energy fluid transfer conduit system in which fluid transfer tubing has a transparent vacuum jacket that includes an intermediate partition between spaced inner and outer walls. The vacuum jacket is thereby divided to define a pair of concentric vacuum chambers on either side of the partition.
Another object of one embodiment of the invention is to provide a collector tube which maximizes the efficiency or reflected solar radiation collection and retention. This is achieved by providing coaxial tubes for the circulating fluid used to collect the reflected solar energy. Thus, the fluid may flow through the innermost tube from one end to other, and then back around the outside of the innermost tube within a second outer coaxial tube. Alternatively, flow may proceed in the opposite direction. In both instances heat radiated from the inner tube is absorbed by fluid in the outer tube. Thermal energy from fluid within the interior tube segment thereby radiates into a surrounding layer of heat collection fluid, thereby conserving thermal energy within the entire system.